The present invention relates to a radiation-curable liquid resin composition for coating and cabling optical fibers, which compositions exhibit superior curing and a reduced amount of particulate matter, for example crystalline inclusions, and produce cured products which exhibit minimum yellowing, exhibit excellent durability. Hence, the liquid curable resin composition which is particularly suitable for use as a coating material coated on optical fibers or optical fiber ribbon matrices. Furthermore, the resin composition when cured can have stable adhesion, and can be easily removed from the material on which the composition has been coated. Therefore, the composition is particularly suitable for use as a soft coating coated on optical fibers.
Radiation-curable compositions are extensively used in the optical fiber industry during the production of optical fibers, ribbons, and cables. For example, optical glass fibers are routinely coated with at least one radiation-curable coating (often two coatings) immediately after the glass fiber is manufactured in a draw tower so as to preserve the pristine character of the glass fiber. Immediately after a coating is applied to the fiber, the coating is rapidly cured by exposure to radiation (commonly ultraviolet light). The industry demands faster production speeds, and therefore, faster curing coating compositions.
Radiation-curable matrix and bundling materials can further support and protect the individual strands of coated fiber as individual strands are bundled together into optical fiber ribbons, optical fiber cables, and associated structures. Also, radiation-curable inks can be used to color code individual strands of optical fiber. All of these types of optical fiber-related materials are preferably radiation-curable and can serve as coating and/or cabling materials.
Two radiation-curable coating layers are routinely employed in optical fiber production before ribboning and cabling. One layer is a soft inner primary coating (xe2x80x9cprimary coatingxe2x80x9d), which directly contacts the glass fiber and prevents microbending. The other layer is a tougher outer primary coating (xe2x80x9csecondary coatingxe2x80x9d), which provides a more durable exterior for the glass fiber.
Examples of radiation-curable primary coatings are disclosed in U.S. Pat. No. 5,336,563 to Coady et al and secondary coatings in U.S. Pat. No. 4,472,019. Additional aspects of optical fiber coating technology are disclosed in, for example, U.S. Pat. Nos. 5,595,820 to Szum, 5,199,098 to Nolan et al.; 4,923,915 to Urruti et al.; 4,720,529 to Kimura et al.; and 4,474,830 to Taylor et al.
Along with expanding installation of optical fiber cables in recent years causing optical fibers to be exposed under various environmental conditions, higher durability is demanded of soft coatings, hard coatings, coloring materials, and bundling materials for optical fibers. Particularly, a resin used as a soft coating must exhibit stable adhesion to glass over an extended period of time. In addition, expanded use of the optical fiber ribbon structure consisting of a plurality of optical fibers requires improvement in the operation for splicing optical fibers. Specifically, it is strongly desired that all materials in optical fiber ribbons such as soft coatings, hard coatings, coloring materials, ribbon matrixes, and bundling materials be removed in one operation from optical fiber glass. Furthermore, as the demand for optical fibers increases, improvement in the productivity of optical fibers is desired. Specifically, it is strongly desired that the materials have characteristics whereby they may be coated onto glass fibers and cured quickly to constantly produce a coat with an even thickness.
To maximize cure speed in an ultraviolet light cure, at least one photoinitiator is required (photoinitiator may be omitted in an electron beam cure). Several photoinitiators can be used to achieve a suitable balance of surface and through cure. Conventional classes of photoinitiators have been used. Mono-acyl phosphine oxide type photoinitiators can be used such as Lucirin TPO [(2,4,6-trimethylbenzoyl) diphenyl phosphine oxide [commercially available from BASF] which exhibits relatively fast cure speed. However, use of commercial Lucirin TPO can cause undesired crystallization effects in the coatings (e.g., during aging), which can result in inclusions and loss of optical clarity (detected under a light microscope). Attempts have been made to use purified Lucirin TPO, but the purification steps are costly. Other phosphine oxide photoinitiators (e.g., CGI 403, Ciba) can show reduced amounts of harmful crystallization effect, but they may also have slower cure speed. Hence, a need exists to discover photoinitiators which can provide both fast cure speed and good optical clarity. The art, until now, has not provided direction on how to achieve such properties.
In addition, photoinitiators can cause yellowing, particularly during long term aging of cured compositions under photolytic aging conditions (e.g., UV or fluorescent light). Heat may also induce yellowing. Discoloration in general and yellowing in particular is undesired and has become anathema in the industry. Hence, a photoinitiator which would provide for lack of harmful crystalline effects and fast cure, but would result in yellowing, would not sufficiently meet the most stringent industry demands.
The art has not recognized a photoinitiator which provides an excellent balance of these properties. For example, a very large number of phosphine oxide photoinitiators are disclosed in, for example, U.S. Pat. Nos. 5,218,009 to Rutsch et al. and 5,534,559 to Leppard et al. However, these patents do not suggest that any particular species of photoinitiators would solve the above-noted problems and provide an excellent balance of properties. Hence, they do not anticipate or suggest the present invention.
The characteristics required for radiation curable resins used as the coating materials for optical fibers include: being a liquid at ordinary temperatures and having a sufficiently low viscosity to be excellently coated; providing good productivity at a high curing rate; having sufficient strength and superior flexibility; exhibiting very little physical change during temperature changes over a wide range; having superior heat resistance and superior resistance to hydrolysis; showing superior long term reliability with little physical changes over time; showing superior resistance to chemicals such as acids and alkalis; exhibiting low moisture and water absorption; exhibiting superior light resistance showing the least discoloration over time; exhibiting high resistance to oils; and producing little hydrogen gas which adversely affects optical fiber characteristics.
High durability and productivity are required for optical fiber along with expanding installation of optical fiber cables in recent years. Specifically, if the durability of a coating material for optical fiber is low and, for example, coloring is conspicuous, the visibility of a color painted for distinguishing optical fiber decreases, exhibiting a problem that the maintenance of the cable is hindered. It has been already confirmed that the reduction of visibility is caused not only by coloring of the bundling material, but also by coloring of the hard coating or the soft coating.
Moreover, an increased demand of optical fibers makes it necessary for the coating materials to cure quickly in a stable manner. A photo-initiator which decomposes fast must be used for the coating materials to cure quickly.
Furthermore, the resin used as the soft coating must exhibit stable adhesion to glass over a long period of time. In addition, when all coating materials are removed from the ribbon structure of optical fibers, there should be no residue remaining on the glass.
Japanese Patent Application Laid-open No. 190712/1989 discloses a composition comprising an acyl phosphine oxide as a photo-curable resin composition which realizes high productivity in fast curing. However, this composition is not necessarily cured at a high enough rate to sufficiently increase the productivity of optical fibers while maintaining the characteristics required for an optical fiber coating material.
Another composition comprising a bis-acyl phosphine oxide has been proposed in Japanese Patent Application Laid-open No. 259642/1996 as a photo-curable resin composition which shows high productivity by being cured at a high rate. However, the bis-acyl phosphine oxide containing a long chain aliphatic group disclosed in this Japanese Patent Application has a poor solubility in resin compositions, and hence cannot be dissolved in the resin compositions in an amount sufficient to ensure a high cure rate.
A decrease in adhesion strength between an optical fiber coating material and optical glass fiber over time is known to induce deterioration in the low attenuation of light signal characteristics of the optical fiber. Although various means have been proposed to stabilize the adhesion strength, none has exhibited a sufficient effect. For example, a method for increasing the adhesion strength of a resin composition to glass under high humidity and high temperature conditions by incorporating an organic functional silane additive is disclosed in Japanese Patent Application Laid-open No. 21546/1992. This method is known to achieve the intended objective only insufficiently because the organic functional silane compound decreases the curing speed of the coating material itself and, hence, can be added in only a limited amount.
The importance of the process for removing all coating materials in ribbon structure optical fibers in one operation is increasing in recent years. In this regard, Ching-Kee Chien et al. reported that the capability of removing all coating materials in one operation largely depends on the temperature when the coating materials are removed and the removal speed (Proceedings of 45th International Wire and Cable Symposium p 554 (1996)). The detailed mechanism is not known at the present time. In addition, no proposal for improving these problems from the aspect of coating material has surfaced up to the present time.
What is needed in the optical fiber art, but what the art seemingly has not yet provided, is a radiation-curable composition which provides fast cure speed, and which, upon cure provides good optical clarity, and minimal discoloration and yellowing. Without this combination of properties, the composition is not meeting the stringent industry demands.
An object of the present invention is therefore to provide a radiation-curable liquid resin composition for coating optical fibers exhibiting superior curing and producing cured products which encounter yellowing only to the least extent and exhibit excellent durability. A further object of the invention is to provide a radiation-curable liquid resin composition which, after being cured, can be easily removed from the material on which the composition has been coated.
The present invention provides a radiation-curable composition for optical fiber coatings comprising about 10 wt. % to about 90 wt. % of at least one radiation-curable oligomer (A), about 10 wt. % to about 90 wt. % of at least one radiation-curable monomer diluent (B), and an effective amount of at least one photoinitiator (C) represented by formula (1): 
wherein Ar1 to Ar3 independently represent unsubstituted and/or substituted aromatic groups, said substituted groups may include among other groups hetero groups comprising O, S and/or N.
The invention further provides covering layers for optical fibers such as radiation-curable inner primary coatings, outer primary coatings, matrix materials, bundling materials, inks, and other types of materials employed in optical fiber technology which comprise the photoinitiator represented by formula (1).
The present invention further provides a light transmitting device comprising an optical fiber preferably an optical glass fiber, and at least one radiation-cured covering layer thereon, wherein the covering layer, before radiation cure, is a radiation-curable composition comprising about 10 wt. % to about 90 wt. % of at least one radiation-curable oligomer, about 10 wt. % to about 90 wt. % of at least one radiation-curable monomer diluent, and an effective amount of at least one photoinitiator represented by formula (1).
The invention further provides a method of making coated optical fibers comprising coating an optical fiber with a radiation-curable composition comprising about 10 wt. % to about 90 wt. % of at least one radiation-curable oligomer, about 10 wt. % to about 90 wt. % of at least one radiation-curable monomer diluent, and an effective amount of at least one photoinitiator represented by formula (1).
The present invention further provides a method of cabling coated optical fibers with a radiation-curable composition comprising about 10 wt. % to about 90 wt. % of at least one radiation-curable oligomer, about 10 wt. % to about 90 wt. % of at least one radiation-curable monomer diluent, and an effective amount of at least one photoinitiator represented by formula (1).
The invention further provides a method for improving the clarity by reducing the amount of particulate matter in a radiation-cured optical fiber coating comprising the step of formulating a radiation-curable composition consisting essentially of about 10 wt. % to about 90 wt. % of at least one radiation-curable oligomer, about 10 wt. % to about 90 wt. % of at least one radiation-curable monomer diluent, and an effective amount of at least one photoinitiator represented by formula (1). Coating ingredients should be excluded from the composition which, upon cure, cause crystalline inclusions. Preferably, Lucirin TPO photoinitiator should be used minimally (for example, less than about 0.1 wt. %) or not at all.
Radiation-curable liquid resin composition which are used preferably for inner primary coatings of glass optical fibers comprise (a) 40-95% by weight of a polyurethane containing a polyoxyalkylene structure comprising at least one oxyalkylene group having 2-10 carbon atoms and an ethylenically unsaturated group, (b) 3-50% by weight of a mono-functional (meth)acrylate of which the homopolymer has a glass transition temperature of less than 20xc2x0 C., and (c) 0.05-10% by weight of a photo-initiator which is a compound having a chemical structure with the following formula (1), as defined above.